Conventionally, various types of optical elements or apparatuses, which are designed to guide and scatter incident light at the same time and let it out in multiple directions, have been publicly known.
One type of such known optical elements or apparatuses configures a surface type light source, in which light is allowed to come in through a side of an extending plate-shaped transparent material, with a reflecting element installed on one surface side, and the vicinity of the other surface provided with light diffusivity to obtain a light outgoing surface, thereby enabling the apparatus to be used as a backlight source or the like for a liquid crystal display.
This type includes, for example, those described in the Published Unexamined Japanese Patent Application No. 62-235905, the Published Unexamined Japanese Patent Application No. 63-63083, the Published Unexamined Japanese Patent Application No. 2-13925, and the Published Unexamined Japanese Patent Application No. 2-245787.
In surface type light sources employing those light scattering light guide apparatuses, light scattering is not volumetrically caused inside a transparent body; instead, the light is merely spread in a light outgoing direction by utilizing irregular reflection or mirror reflection in the vicinity of the surface of the transparent body or in a reflecting element, and so, it cannot necessarily be said that such light scattering light guide apparatus is capable of providing a high light diffusivity.
In addition, when an attempt is made to obtain a surface type light source, which receives light from a side and gives uniform illuminance, it is necessary to provide the reflectivity or the like of a reflecting element with some gradient as may be easily understood from the examples shown in the known documents mentioned above. This unavoidably results in a complicated and large (especially thick) structure of the light scattering light guide apparatus.
Accordingly, in using this type of light scattering light guide as the light source or the like of a backlight for a liquid crystal display device, which is required to have a uniform and high illuminance and a thin structure, various additional configuring means are necessary to reinforce light scattering and assure uniform illuminance for a surface type light source. Incorporating such light scattering power reinforcing means, however, used to conflict with the requirement for a thin structure.
Another type of known optical elements or apparatuses has a light diffusing plate which consists of an extending plate-shaped transparent material with a granular substance dispersed therein, the granular substance having a different refraction index from that of the transparent material.
This type includes, for example, those disclosed in the Published Unexamined Japanese Patent Application No. 1-172801, the Published Unexamined Japanese Patent Application No. 1-236257, the Published Unexamined Japanese Patent Application No. 1-269901, the Published Unexamined Japanese Patent Application No. 1-269902, and Published Unexamined Japanese Patent Application No. 2-221925.
The aforesaid Published Unexamined Japanese Patent Application No. 2-221925 discloses a configuration in which light is allowed to let into a light diffusing plate through its side, one surface side being provided with a reflecting element and the other surface functioning as the light outgoing surface, thus configuring a backlight source or the like of a liquid crystal display.
In those light diffusing plates, light scattering is volumetrically caused by nonuniformity in refraction index due to a granular substance dispersed and mixed in the transparent body. In this sense, it can be said that they are capable of improving the light diffusion efficiency in comparison with the aforesaid first type, however, they have the following problems.
They have a drawback such that it is extremely difficult in practice to uniformly disperse a granular substance of a different type in the base substance. This will cause, irregular distribution in an inhomogeneous structure to hinder uniform light diffusion.
For instance, when particles measuring a few microns are mixed in a methylmethacrylate (MMA) monomer for polymerization, uniform light scattering power cannot be obtained because of uneven distribution of particles caused by a difference in density or compatibility. For the same reason, it is also difficult to obtain a structure of highly uniform particle dispersion when particles are incorporated in a polymer such as polymethyl methacrylate (PMMA) at a high temperature.
Besides, the basic unit of the structure with an irregular refraction index that provides light scattering is limited to the granular form; however, this is not desirable from a viewpoint that light scattering is to be caused by uniformly and effectively using the whole volume of a light scattering light guide. Especially, when a surface light source of a relatively large area with a uniform illuminance such as one applicable to the backlight source for a liquid crystal display has to be obtained, the particle dispersion density will become lower, so that the whole of the light scattering light guide will not be able to contribute to uniformity of light scattering. This is not desirable in obtaining highly uniform diffused light.
To distribute the structure with an irregular refraction index throughout the light scattering light guide without changing the light scattering power of the whole light scattering light guide, it can be considered effective to reduce the diameter of the particles to be dispersed and to increase the number of particles. It is not necessarily easy, however, to fix the particle diameter of a specified material to an extremely small value primarily because of the limitation of the manufacturing technology of minute particles. Furthermore, excessively small particle diameter may cause scattering to become dependent on wavelength and resulting undesirable coloring phenomenon.
In the aforesaid Published Unexamined Japanese Patent Application No. 2-221925, PMMA is dissolved in a mixed monomer of MMA and MA (methyl acrylate) to prepare a methacrylate resin syrup. Then an inorganic filler such as glass beads and ground quartz which has a specific gravity exceeding 2 and a different refraction index from the methacrylate resin is dispersed and mixed, and an initiator of polymerization is added to cause polymerization reaction (copolymerization reaction) to take place, thus acquiring a light scattering light guide. In this case, light scattering also practically depends solely on the inorganic filler; therefore, it is not intended at all to generate a structure with an irregular refraction index in the course of the polymerization reaction process. In the combined copolymerization (e.g., 50 wt % methyl methacrylate and 50 wt % ethyl methacrylate, or 50 wt % methyl methacrylate and 50 wt % methyl acrylate) described in the specification of the publication of unexamined JP patent application No. 2-221925, no structure with an irregular refraction index is generated because of an extremely good compatibility. Accordingly, the resin produced from the polymerization reaction functions only as a transparent base material, and does not contribute to light scattering.
Conventionally, various shapes of light scattering light guides are known in addition to a plate-shaped one. For instance, in a light irradiating apparatus used for ocean farming or for the cultivation of chlorella, outgoing light which covers a wide angle range is obtained by providing, for example, a scratched area on the surface of a rod-shaped or linear light guide so that the scratched area will cause light scattering. In such a method, however, scattered light is concentrated on the scratched area, making it difficult to obtain a light flux with an adequate level of uniformity and a large cross-sectional area.
Further, a fiber light guiding means is used to introduce sunlight into a building for the purpose of interior lighting; however, this presently requires expensive and large auxiliary equipment for producing outgoing light fluxes which is good enough to illuminate a large space efficiently and uniformly.
Thus, the publicly known light scattering light guides present various problems such as those described above, and a light scattering light guide which solves those problems has been waited in many optical fields including a field requiring a bright surface type light source with a high level of uniformity, which is typically represented by the backlight source for a liquid crystal display.